In dental treatment, dental prostheses each having a complicated shape that is adaptable to each patient have been used, and among the dental prostheses, dentures have widely been used. Among the dentures, a so-called partial denture (hereinafter, also simply referred to as “denture”) has metal attachments called as clasps and has a structure by which the clasps are fixed to health teeth in such a way that the clasps are hooked to the health teeth. Therefore, there has been an irrational problem that a part of each health tooth to be a grounding surface of the clasps usually has to be cut in order to wear the partial denture. Furthermore, there has also been a problem that the health of the teeth that would be healthy is liable to be impaired because of cutting the teeth or hooking the clasps to the teeth. Moreover, there has been another problem about aesthetic quality that a clasp part is seen by a conversational partner or the like to be found that a person in question wears a denture. The problem about the aesthetic quality is an important problem that should be taken seriously to a person who has to use a denture because the problem can lead to mental illness.
In order to solve these problems once for all, a denture without a clasp which is called a “non-clasp denture” and is excellent in aesthetic quality, the denture being supported by a “fixing part” that substitutes for the clasps and is manufactured from the same material (synthetic resin) as has been used for manufacturing gums for conventional dentures has been developed in recent years, and dentists who handles the “non-clasp denture” have been increasing.
As description will be made later about an outline of manufacturing the “non-clasp denture”, when the “non-clasp denture” is manufactured, it is necessary that the denture be manufactured with which a problem of adaptability or occlusion does not arise by manufacturing the “fixing part” that can substitute for the conventional clasps with a synthetic resin. Since the denture chews various kinds of foods with strong force and is used in oral cavity where there is a possibility that the denture is exposed to a gastric acid, polycarbonate resins or the like that are excellent in various properties such as shock resistance have been used as a synthetic resin for manufacturing the fixing part. In the “non-clasp denture”, the “fixing part” of the denture is manufactured using a gypsum mold with a polycarbonate resin or the like, and the synthetic resins have a property that contraction occurs during setting (polymerization). Therefore, a teeth model having dimensions for which the contraction of the synthetic resins that occurs during polymerization is taken into consideration in advance is considered to be required in order to manufacture the “non-clasp denture” that is excellent in dimensional accuracy and has no problem in occlusion. Specifically, a reproduction model the size of which is adjusted to somewhat larger than the size of a teeth model which is made of gypsum and is manufactured faithfully based on an impression of teeth taken from a patient in a dentist's office has been manufactured.
For example, the polycarbonate resin that has been used in manufacturing the “non-clasp denture” as a material for manufacturing a gum part including the “fixing part” that substitutes for the clasps is a material in which a large heat contraction (about 0.4 to about 0.7%) occurs during polymerization. Therefore, when the “non-clasp denture” is manufactured using a teeth model made of gypsum as it is, the teeth model manufactured based on an impression actually taken from a patient, the “non-clasp denture” becomes too small and the adaptability becomes poor. The “adaptability” is directly linked to the “occlusion” that is an extremely important element among the properties of the denture to be set and used in oral cavity having keen senses. Furthermore, when the “non-clasp denture” is too small, it is impossible to repair it and it is necessary to remake another one because the “non-clasp denture” has a fatal defect. Therefore, in manufacturing the “non-clasp denture”, it is an extremely important matter how to manufacture the aforementioned teeth model in a precise size, the teeth model (hereinafter, also simply referred to as “reproduction model”) reproduced so that the size is adjusted to somewhat larger than the size of the teeth model which is made of gypsum and is manufactured using an impression of teeth taken from a patient with the contraction of the resin taken into consideration.
The reproduction model is manufactured by using a high-expansion gypsum composition exhibiting the expansion coefficient higher than that of a gypsum material for manufacturing a usual gypsum model. In addition, use of the gypsum composition having such a large expansion coefficient is not only limited to the case where the above-described “non-clasp denture” is manufactured, but also, for example, the gypsum composition having such a large expansion coefficient is used for manufacturing a so-called full denture (hereinafter, referred to as “artificial teeth”) in which a dental plate is manufactured using a resin, and the gypsum composition is widely used for preparing gypsum molds. In the present invention, description is made by taking a case where the “non-clasp denture” is manufactured as a representative example. The reason is that, in this case, since a higher adaptability is required in a sense that the delicate occlusion is required, it becomes necessary to more precisely control the expansion coefficient of the high-expansion gypsum composition particularly according to the contraction of a resin to be used in manufacturing the above-described reproduction model.
Under the above-described circumstances, various high-expansion gypsum compositions in which the expansion coefficient in usual gypsum materials is enhanced in order to deal with the contraction of a resin have so far been proposed. For example, as the gypsum for making an artificial tooth, there has been a proposal (see, Patent Literature 1) on high-expansion gypsum in which the expansion coefficient is enhanced utilizing heat expansion of a resin by adding a granular resin to the gypsum, and there has also been a proposal (see, Patent Literature 2), as a composition that can be used as a material for a dental purpose model, in which a high expansion coefficient with a linear expansion of at least 0.5% can be realized during coagulation or setting by adding a silicate such as silica sol as an inorganic additive to gypsum plaster.